PHILADELPHIA (CBS) – There is a new warning about azithromycin, a medicine that was prescribed to 40 million people last year.

On Tuesday, the FDA announced they updated the warning labels on the antibiotic to include fatal heart problems in some patients.

The antibiotic azithromycin is sold under the brand names Zithromax and Zmax. It’s also commonly called a Z-Pak and is prescribed for a variety of bacterial infections including pneumonia, sinusitis, skin and ear infections.

The new FDA warning says the antibiotic could cause an irregular heart rhythm in some patients that can be deadly.

“The group of patients for whom this warning is most important are those who already have some cardiovascular problem,” said Dr. Wayne Ray, a researcher with the Vanderbilt University School of Medicine.

The azithromycin danger was first noted last May in the New England Journal of Medicine in a study that found the drug had a higher rate of cardiovascular death when compared to other antibiotics. The FDA warning follows a review by its medical team that included additional research.

The new advisory says doctors should be aware of the risk factors before prescribing the drug.

“Such as a prior heart attack, or angina, or chest pain or people who are known to have abnormalities in their heart rhythm,” said Dr. Ray.

The FDA also noted that other antibiotics in the same class have the potential for the same problem, so doctors need to consider all the risks.

Pfizer, the maker of the medication, says it regularly monitors the safety and efficacy of its products. In collaboration with the FDA, it has updated the medication label to include the risk factors, which won’t impact the majority of people.